An array of disk drives referred to as a Redundant Array of Independent Disks (RAID) is typically employed in enterprise systems to store large amounts of data to achieve redundancy and/or improved performance through command striping. With command striping, a large host command is divided into a number of smaller access commands by an array controller, and the smaller access commands transmitted to a number of the disk drives in the array for concurrent processing. The array controller will typically not report a command completion to the host until each of the individual drives have completed their assigned access commands corresponding to a host command.
Each disk drive in an arrayed storage system may implement command queuing wherein a number of access commands received from the array controller are queued in a command queue, and then selected for execution in an order that minimizes the access latency of the disk drive in terms of seek latency and rotational latency. This type of command scheduling implemented internal to each disk drive is typically referred to as rotational position optimization (RPO). If a number of access commands are queued in the command queues that correspond to a number of different host commands, there may be a large variance in the command completion time as seen from the host since each disk drive selects the access commands from the command queue independent of the other disk drives. That is, each disk drive executes the RPO scheduling algorithm independent of the other disk drives which can lead to a large variance in completing the access commands of any one host command.